In the past most electric motors were constructed according to slotted designs in which the copper conductors of the motor winding were placed between teeth in slots of the laminated iron rotor or stator structure. The slotted designs provide a motor with a relatively small air gap to achieve a desired high permeance. The recent advent of high energy permanent Nd Fe B (neodymium, iron, boron) magnets has made slotless designs feasible in all motors, particularly high performance servo motors. See, for example, U.S. Pat. No. 4,954,739, Servo Motor With High Energy Product Magnets, by Roy D. Schultz et. al. granted to Kollmorgen Corp. In the slotless designs the copper windings are located in the air gap rather than in the slots.
Slotless motor designs have great potential advantages over conventional slotted designs. The slotless designs have a higher potential level of efficiency due to extremely low eddy current losses, extremely low hysteresis losses, the absence of cogging losses and the lack of appreciable iron losses. High speeds in the range of 40k-120k rpm are readily attainable. The operation can be perfectly smooth over a wide speed range since there is no cogging due to the absence of teeth in the slotless design. Furthermore, the lack of magneto-strictive noise from the teeth of a normal design allows for very quiet operation. The slotless design is also capable of a faster motor response (acceleration/deceleration) due to a low inductance.
Various methods have been proposed for making the slotless motors. For example, in the aforementioned U.S. Pat. No. 4,954,739, the winding is formed using a cylindrical support with a reduced diameter portion at one end. A fiberglass sleeve is placed around the uniform diameter portion and thereafter preformed coils are placed in position. When the coils are in place, the thicker end turn portions at one end of the winding are flared inwardly at the reduced diameter portion of the support and the other end of the winding is flared outwardly. The winding can then be inserted into the cylindrical back iron shell starting with the inwardly flared end of the winding. The support can thereafter be withdrawn from the outwardly flared end leaving the fiberglass sleeve as part of the motor structure. The winding is encapsulated using a suitable resin after the winding is inserted into the stator shell.
U.S. Pat. No. 4,130,769 issued Dec. 19, 1978, to Karube describes a layered preformed winding technique for making a slotless, brushless DC motor. A fixture is utilized for preforming flat single layered coils. The preformed coils are placed in the back iron cylinder in a shingle layered fashion with one of the straight coil portions in an outer layer against the back iron and the other straight coil portion in an inner layer. This design appears to be limited to windings with a two conductor thickness.
U.S. Pat. No. 4,563,808 "Methods of Producing Slotless and Toothless Wound Stator" issued to Robert Lender on Jan. 14, 1986, (now assigned to the assignee of this application) describes a method of making and utilizing a fixture which attaches to a cylindrical stator housing, having a smooth circumference wall. Once the stator is placed in the fixture, temporary fingers are extended radially inwardly. The coils are then wound around the fingers which form temporary slots similar to those in slotted motor designs. The conductors are thereafter forced outwardly against the inner wall of the back iron cylinder by a non-magnetic, expandable, reformable plastic cylinder.
U.S. Pat. No. 4,645,961 "Dynamoelectric Machine Having A Large Magnetic Gap and Flexible Printed Circuit Phase Winding" issued to Herbert Malsky on Feb. 24, 1987, describes the use of a long flexible printed circuit board insulated on both sides. The printed circuit board is rolled in "jelly roll fashion" and placed in the stator back iron cylinder. The winding is closed by soldering in connecting wires at appropriate locations.
In order to fully take advantage of the slotless motor design characteristics the active conductor portion of the winding in the air gap must be straight and parallel to each other and, preferably, to the motor axis of rotation. Any deviation from the parallel alignment decreases the rotatory torque produced by the current flow through the conductor and, hence, reduces the efficiency of the motor. Furthermore, failure to achieve parallel alignment of the conductors results in a larger then necessary air gap and a reduced thermal conductivity for removing heat from winding. As a result of one or more of these factors, the past motor designs and methods of production have not been able to achieve the full potential of the slotless design at a reasonable cost.